1. Field of the Invention
The present invention relates to a method of etching silicon (Si) wafer preferable in forming an Si diaphragm used for a semiconductor pressure sensor, a semiconductor acceleration sensor or the like and relates to an Si wafer having a diaphragm of an octagonal shape.
2. Description of Related Art
An explanation will be given of a conventional method of forming an Si diaphragm used for a semiconductor pressure sensor, a semiconductor acceleration sensor or the like in reference to FIGS. 56 and 57. First, as shown in FIG. 56, gages (strain gages) 2 having a predetermined shape are formed on a lower face of an Si wafer 1. Further, an etching mask 3 composed of SiO.sub.2 or SiN is formed on an upper face of the Si wafer 1. Successively, as shown in FIG. 57, the lower face side of the Si wafer 1 is pasted on a ceramic substrate 5 via a protecting member 4 of wax or the like. Thereby, the lower face side of the Si wafer 1 is protected.
As shown by FIG. 57, the Si wafer 1 and the ceramic substrate 5 are immersed in an anisotropic etching solution composed of, for example, an aqueous solution of KOH stored in a vessel 6 to thereby carry out chemical etching. In this case, a plurality of sheets of the Si wafers 1 (as well as the ceramic substrates 5) are set in a carrier 8 and immersed into the anisotropic etching solution 7 along with the carrier 8.
When the Si wafer 1 is immersed into the anisotropic etching solution 7, as shown by two-dotted chain lines in FIG. 56, etching faces in correspondence with opening portions 3a of the etching mask 3 are dissolved, and recess portions 9 are formed. Further, the bottom portions of the recess portions 9 constitute a diaphragm 9a. In this case, the anisotropic etching is carried out by the anisotropic etching solution 7 and accordingly, sharp corners are formed at end portions 9b of the diaphragm 9a. When the end portions 9b of the diaphragm 9a are sharp corners, the pressure resistant strength of the diaphragm 9a is lowered.
Hence, conventionally, the Si wafer 1 having the recess portions 9 formed is immersed in an isotropic etching solution composed of, for example, an acid-base etching solution and the inner faces of the recess portions 9 are isotropically etched by which a processing of rounding the end portions of the diaphragm 9a is executed.
However, the isotropic etching processing is a diffusion controlled reaction and therefore, control of the reaction is difficult. Further, in the acid-base anisotropic etching solution, ageing change of composition is considerable. Therefore, there arises a problem where the depth dimension of the recess portion 9 formed in the Si wafer 1, that is, the thickness dimension of the formed Si diaphragm 9a is significantly dispersed. Furthermore, there causes a drawback where the inner side bottom face of the recess portion 9, that is, the surface of the Si diaphragm 9a is considerably roughened or a drawback where dimension D1 of the diaphragm specified in FIG. 17 is varied. This is because, when a face having crystal orientation of (110) of the Si wafer 1 (hereinafter, referred to as (110) face) is etched by the anisotropic etching solution 7, impurities such as metal ions (for example, Pb) included in the anisotropic etching solution 7 by a very small amount are adsorbed on the etching face of the Si wafer 1 and the etching rate is varied by masking operation due to the adsorption of impurities.
Further, an example of a diaphragm for a semiconductor pressure sensor is disclosed in Japanese Patent Application Laid-Open No. H.4-119672. According to the example, an Si diaphragm having an octagonal shape is formed by etching an Si wafer having crystal orientation of (110). A specific explanation will be given of a method of forming this Si diaphragm in reference to FIGS. 58 through 65.
First, as shown by FIGS. 58 and 59, an etching mask 102 composed of SiN, SiO.sub.2 or the like is formed and patterned on an upper face (left face in FIG. 59) of an Si wafer 101 having crystal orientation of (110). It is to be noted that gages, electrodes or the like having predetermined shapes have previously been formed on a lower face (right face in FIG. 59) of the Si wafer 1. Then, anisotropic etching is carried out by immersing the Si wafer 101 in an alkaline etching solution composed of KOH or the like. Thereby, an Si diaphragm 103 having an octagonal shape shown in FIGS. 60 and 61 is formed.
The Si wafer 101 in a state that the etching mask 102 is removed from the Si wafer 101 which has been finished with etching, is shown in FIGS. 62 through 65. As shown in FIG. 62, the Si diaphragm 103 having an octagonal shape is surrounded by two upper and lower (100) faces, two left and right (111) faces and four (111) faces at oblique positions. Further, as shown in FIG. 63, the two upper and lower (100) faces constitute inclined faces inclined relative to a diaphragm face 103a of the Si diaphragm 103 and an angle of inclination thereof (that is, angle produced by intersecting the (100) face with the diaphragm face 103a) is 45.degree..
Also, as shown in FIG. 64, the two left and right (111) faces constitute inclined faces inclined relative to the diaphragm face 103a and an angle of inclination thereof (that is, an angle produced by intersecting the left or right (111) face with the diaphragm face 103a) is 35.degree.. By contrast, as shown by FIG. 65, the four (111) faces disposed at the oblique positions constitute faces vertical to the diaphragm face 103a and an angle of inclination thereof (that is, an angle produced by intersecting the four (111) faces at the oblique positions with the diaphragm face 103a) is 90.degree..
According to the example described above, the two upper and lower (100) faces and the two left and right (111) faces intersect with the diaphragm face 103a by small angles (45.degree., 35.degree.) whereas the four (111) faces at the oblique positions are orthogonal to the diaphragm face 103a. Accordingly, when excessive pressure is applied on the diaphragm 103, stress is concentrated at portions where the four (111) faces intersect the diaphragm face 103a. Thereby, the above-described portions may be destructed.